docs: def validate_output_format(self, default_format):
          """ Prepare output format for later use.
          """
          output_format = self.options.output_format
  
          # Use default format if none is given
          if output_format is None:
              output_format = default_format
  
          # Check if it's a custom output format from configuration
          # (they take precedence over field names, so name them wisely)
          output_format = config.formats.get(output_format, output_format)
  
          # Expand plain field list to usable form
          if re.match(r"^[,._0-9a-zA-Z]+$", output_format):
              self.plain_output_format = True
              output_format = "%%(%s)s" % ")s\t%(".join(formatting.validate_field_list(output_format, allow_fmt_specs=True))
  
          # Replace some escape sequences
          output_format = (output_format
              .replace(r"\\", "\\")
              .replace(r"\n", "\n")
              .replace(r"\t", "\t")
              .replace(r"\$", "\0") # the next 3 allow using $() instead of %()
              .replace("$(", "%(")
              .replace("\0", "$")
              .replace(r"\ ", " ") # to prevent stripping in config file
              #.replace(r"\", "\")
          )
  
          self.options.output_format = formatting.preparse(output_format)